In a typical radar system, a radar antenna radiates a signal in the direction of an object which is the subject of the radar inquiry. A portion of this energy is reflected from the object back toward the radar system which receives and processes this reflected energy to extract information regarding the object. For example, in relatively simple systems, the relative velocity between the object and the radar system can be determined in accordance with the Doppler shift between emission and return of the signal. In more complex radar systems, signal processing techniques performed on the reflected signal may yield data regarding the size, shape, range, and direction of the object.
In some radar applications, the object may carry radar jamming systems which detect the emitted signal, modify it in one or more ways known to those skilled in the art, and retransmit the modified signal so as to deceive the radar system. A radar signal that is deterministic and periodically repeats, is more vulnerable to deceptive jamming than one that never repeats in time.
For example, a Target Detection Device (TDD), sometimes referred to as a fuze, is commonly found in a guided missile. Many TDD determine the distance between the missile and the target, and when the missile reaches a predetermined distance to the target, the TDD detonates the missile warhead to achieve maximum impact on the target. In such applications, the targets may be equipped with radar jamming systems such as digital radio frequency memory systems (DRFMs) which store the incoming radar signal in memory and determine the repeat interval of the signal. The DRFM then emits a signal replicating the return signal expected by the radar system back to the TDD. The signal emitted by the DRFM is delayed to apparently arrive before the reflected signal would normally arrive for the given distance between the radar system and the DRFM. This signal deceives the radar system into determining that the object is closer than it actually is.
Existing TDDs use repetitive waveforms to enable range determination. One example of waveform modulation is a pseudo-random noise sequence. In the pseudo-random noise sequence, the radar system emits binary sequences characteristic of a noise waveform, but which is repeated after a predetermined time interval. Another modulation format is frequency modulation continuous wave (FMCW), also referred to as swept-frequency or chirp waveforms. Yet another modulation format is medium pulse repetition rate. In a medium pulse repetition rate radar, the transmit waveform modulation is a train of pulses, and the range to an object is determined by the delay between transmission and reception of the pulse.
The above-described waveforms are deterministic. The deterministic characteristic enables intelligent targets that carry repeater jammers to store the TDD signal, delay it beyond the period where the signal repeats itself, and retransmit the delayed signal back toward the TDD delayed so that the target appears closer to the TDD than it actually is. The TDD in such instances typically detonates the missile warhead at a range beyond the lethal radius of the weapon.
One particular radar system employs TDDs having multiple radio frequency phases. For example, in one multiple radio-frequency phase missile application, a 255-bit or 511-bit, maximal-length sequence, pseudo-random waveform modulation is used to detect the range from the missile to the target. This pseudo-random code bi-phase modulates the radio frequency (RF) carrier. The modulation of the signal returned from the target is correlated with delayed images of the originally emitted code. A correlation occurs when the delay is equivalent to twice the target range. Samples of the correlated output are then processed by standard signal processing techniques so that the target is detected.
For example, assume an approximate signal propagation velocity of one foot per nanosecond and a straight line two-way travel path. If the time delay of a single bit of the code modulation is ten nanoseconds (ns) then a delay of one bit in the returned signal would indicate a distance to the target of five feet. Similarly, five bits or 50 nanoseconds of delay indicates a range of 25 feet, and a 255-bit delay indicates a range of 1275 feet. This range for a 255-bit, 1275 feet, is called the unambiguous range. Delays beyond 255-bits fall into an ambiguous range. For example, a delay of 256-bits indicates a range of 5 feet because the periodic nature of the 255-bit code. Thus, if a radar jammer can store the repetitive waveform, amplify it, and transmit the waveform back with the proper delay, the target carrying the jammer can be made to look closer in range than it actually is.
Thus, it is an object of the present invention to provide a radar system which emits a random electromagnetic signal at a target in order to provide information regarding the target.
It is a further object of the present invention to provide a radar system which receives a random, electromagnetic signal returned from an object illuminated with a random, electromagnetic signal emitted by the radar system, and provides information about the object.
It is yet a further object of the present invention to provide a radar system which emits an electromagnetic signal modulated by random noise in the direction of a target and receives a reflected electromagnetic signal returned from the object and determine the distance to the object.
It is yet a further object of the present invention to provide a Target Detection Device (TDD), sometimes called a fuze, which uses an electromagnetic signal modulated by random noise directed at an object and receives a reflected electromagnetic signal returned from the object in order to determines the distance to the object.
It is yet a further object of the present invention to provide a Target Detector Device which uses an electromagnetic signal including or modulated by random noise directed at an object and receives a reflected electromagnetic signal returned from the object in order to determine the distance to the object and further determine the velocity of the object relative to the radar.
It is yet a further object of the present invention to provide a Target Detection Device which uses an electromagnetic signal consisting of or modulated by random noise directed at an object and receives a reflected electromagnetic signal returned from the object in order to provide a distance to the object, the velocity of the object relative to the radar and further generate control commands in accordance with the distance and velocity of the object.